The present invention relates to a non-compressible composite packing material for use with a printing blanket in offset lithography, and more particularly, to an improved packing material including a polymeric compound on at least one surface which provides the desired gauge to the printing blanket assembly.
One of the most common commercial printing processes is offset lithography, in which a rotary cylinder covered with a printing plate having an image area receptive to and covered by ink is rotated so that its surface contacts a second cylinder covered with an ink-receptive printing blanket. The ink on the image surface of the printing plate transfers, or offsets, to the surface of the blanket, and paper is passed between the blanket covered cylinder and a back-up cylinder to transfer the image from the surface of the blanket to the paper.
During the step in which the image is transferred from the plate to the blanket and from the printing blanket to the paper, it is important to have intimate contact between the contacting surfaces. This is ordinarily achieved by positioning the cylinders so that there is a fixed interference between the contacting surfaces so that the blanket is compressed throughout the run to a fixed depth. In one known method, this fixed interference is accomplished by inserting one or more non-compressible packing materials between the blanket and/or plate and the surface of the cylinders on which they are mounted to build up the thickness of the blanket and/or plate, providing even gauge and adjusting the pressure necessary to achieve good ink transfer.
Conventional, non-compressible packing materials used under the blanket have comprised relatively thin layers of polyester such as Mylar or calendered paper. However, multiple layers of these packing materials may be required in order to build the surface of the blanket to achieve sufficient interference fit. This causes numerous handling and installation problems and requires additional time for installation. Further, once positioned on the blanket cylinder, these packing materials often tend to slip or slide, which may render the blanket surface non-uniform and result in poor print quality. Non-compressible composite packing materials comprising one or more layers of woven fabric, rubber-based adhesives and rubber-based surface layers have been developed which have a greater thickness than paper packing materials. However, such composite packing materials are expensive to produce due to the traditional manufacturing methods of multiple pass, knife coating of the rubber-based adhesive and surface layers and the high cost of the woven fabrics used. Such composite packing materials frequently require at least two layers of fabric to achieve the necessary low elongation, high tensile strength properties. Further, the woven fabrics used in current composite packing materials are oriented such that the required properties are achieved in only one direction (typically the machine direction). In addition, composite packing materials suffer from gauge loss during use due to the poor compression set properties of the rubber based compounds used and the woven fabrics themselves. Thus, the life of the composite packing materials is reduced as the materials tend to lose gauge (i.e., thickness) during operation.
Accordingly, there is still a need in the art for a low-cost, non-compressible, non-directional packing material having a thickness which is sufficient to be installed without the need for multiple layers, and which maintains its gauge over its useful life.